xDSL technology is a technology which provides high bit-rate data transfer on unshielded twist pairs which is used for telephone. xDSL technology may include standards of Very-High-Bit-Rate Digital Subscriber Line (VDSL) and Second Generation of VDSL (VDSL2), and future standards such as Third Generation of VDSL (VDSL3), Multi-input multi-output VDSL (MIMO-VDSL) and Vectored DSL.
FIG. 1 illustrates VDSL system structure. The VDSL system includes a communication station 1 at central office which is named VTU-O and a communication station 2 at the remote side which is named VTU-R. The Communication station 1 and the communication station 2 communicate via a loop. The signal from station 1 to station 2 is called downlink signal and the signal on the opposite direction is called uplink signal. Station 1 and Station 2 may respectively support multiple modes for operation and each mode uses different frequency band.
VDSL channels vary with regions, and normally a region contains 4-6 channels. FIG. 2 shows an example of a regional frequency plan. According to this frequency plan 2-3 downlink channels and 2-3 uplink channels are provided. The starting frequency of the second downlink channel (DS2) is usually above 5.2 MHz.
FIG. 3 illustrates a scenario in VDSL system. In this scenario, two or more VDSL lines between stations are adjacent to each other in a bundle of lines. According to signal processing theories, crosstalk between loops increases with the increase on frequency. So, in the scenario showed in FIG. 3, crosstalk occurs between two lines. The crosstalk may be more serious in VDSL because VDSL uses a higher frequency.
When VTU-O1 and VTU-R1 are in operation, some processes will be adopted to reduce the crosstalk between VDSL lines. If the length L1 of the Line 1 is relatively short and the configured operating rate is relatively low, for example, a line with the length of 300 m is only configured as 20 Mbps in downlink while attainable rate in downlink of the line may be up to 80 Mbps., the process may be adopted that the power at each frequency point is reduced so as to decrease the system power consumption. In this way, the downlink transmit power of VTU-O1 may be much lower than the rated power. Thus the crosstalk between lines may be reduced to some extent.
In VDSL system, besides the crosstalk may be caused between neighboring lines which are in normal operation state, serious crosstalk may also be caused by the VSDL lines which are in initialization process to neighboring lines, affecting the transmission performance of the VDSL lines in transmitting data.
According to VDSL2 draft standard, the initialization procedure contains 5 phases which are illustrated below in table 1.
TABLE 1G.994.1ChannelTrainingChannelExchangeHandshakeDiscoveryanalysis
During channel discovery phase, central communication station VTU-O firstly sends O-P-Channel-Discovery signal. According to VDSL2 standard, the O-P-Channel-Discovery signal almost covers all the frequency points of each channel (such as the DS1 and DS2 as shown in FIG. 2), and transmit power of the signal is confined as below the rated power.
During the channel discovery phase, situations with the actual transmit power of the O-P-Channel-Discovery signal mainly include:                (1) The O-P-Channel-Discovery signal is directly transmitted that is not treated with DPBO (downlink power back-off);        (2) If the length of line 2 is pretty long, for example, the length is 1.5 km. In this case, even DPBO is enabled; the downlink O-P-Channel-Discovery signal will not be treated with DPBO according to the VDSL2 standard.        
So, the O-P-Channel-Discovery signal is sometimes transmitted by a power close to rated power and the long line L2 of FIG. 3 causes a serious crosstalk upon the high frequency signal of the short line L1.
Assuming the short line L1 is 200 m, O-P-Channel-Discovery signal is transmitted over the long line L2 at rated power, the frequency plan in FIG. 2 is adopted, i.e., DS1 from 138 KHz to 3.75 MHz, and DS2 from 5.2 MHz to 8.5 MHz; the crosstalk from L2 to L1 is shown in FIG. 4. According to FIG. 4, during the channel-discovery phase of the long line, the crosstalk of the DS2 frequency band of the short line is very serious, and at some frequencies the crosstalk power is even higher than the signal power of the short line. This may impose a great impact on the transfer performance of the short line in transferring data. For example, the bit error rate may increase seriously.